1. Field of the Invention
This invention relates generally to dynamic memory devices, and, more specifically, to controlling refresh operations in a dynamic memory device.
2. Description of the Related Art
Many of today's dynamic memory devices, such as Synchronous RAM (SDRAM), double data rate SDRAM devices (DDR SDRAM), Rambus™ DRAM (RDRAM) and the like, require precise timing schemes for proper operation. For example, data stored in a dynamic memory tends to quickly dissipate and may be lost unless it is periodically refreshed. Thus, the timing of these refresh operations is critical to maintaining the integrity of data stored in a dynamic memory device. The timing of these refresh operations, however, is complicated by the fact that variable external factors, such as temperature, affect the rate at which the charge stored in the memory dissipates.
Accordingly, designers of dynamic memory devices commonly take into consideration the range of temperatures at which the devices are intended to operate. Typically, a charge stored in a dynamic memory device operating at a relatively high temperature dissipates more quickly than a charge stored in a dynamic memory device operating at a relatively low temperature. Thus, if the designer selects the refresh rate to accommodate the worst case mode of operation, i.e., relatively high temperature, then the refresh rate should be sufficiently fast for the entire range of operation. That is, the refresh operation should occur sufficiently often to prevent the loss of data from dissipation.
This worst case design technique suffers from at least one significant deficiency—excessive power consumption. For example, a refresh rate that is determined by the upper boundary of operating temperature range may not provide for an efficient refresh rate for normal use in lower temperature ranges. That is, excessive amounts of power may be consumed by performing refresh operations too often.
Power consumption is a significant factor in electronic devices. It is desirable to reduce power consumption when implementing certain applications. In particular, wireless and battery operated equipment require lower power consumption designs to operate for acceptably long periods of time. Also, there is a drive to design smaller and more efficient electronic circuitry for many devices, such as PDAs, wireless telephones, cellular phones, portable computers, portable sensors and a variety of electronic equipment that are generally sensitive to heat buildup. The wasted power caused by excessive refresh operations typically manifests as additional heat, which may need to be removed to permit proper operation of the device. Additional mechanical cooling devices, however, tend to increase the size and cost of the device.
The present invention is directed to overcoming, or at least reducing, the effects of, one or more of the problems set forth above.